1. Field of the Invention
The present invention relates to an image display device, and more particularly to an image display device wherein an electron source and a phosphor screen, which forms an image by irradiation of an electron beam emitted from the electron source, are included in a vacuum envelope.
2. Description of the Related Art
In general, in an image display device wherein an electron beam which is emitted from an electron source is radiated on a phosphor body, thereby causing the phosphor body to emit light and displaying an image, a vacuum envelope accommodates the electron source and the phosphor body. A gas occurring within the vacuum envelope increases pressure within the envelope. Consequently, the amount of electrons from the electron source decreases and high-luminance image display may be disabled. It is thus necessary to maintain the inside of the vacuum envelope at a high vacuum level.
In addition, the gas occurring in the vacuum envelope may be ionized by the electron beam, and the generated ions may be accelerated by an electric field. The accelerated ions may strike and damage the electron source.
In a conventional color cathode-ray tube (CRT), a getter material, which is provided in the vacuum envelope, is activated after sealing, and a gas that is emitted from, e.g. an inner wall at the time of operation is adsorbed on the getter material. Thereby, a desired vacuum level is maintained. Attempts have been made to apply such a vacuum level increase and vacuum level maintenance by the getter material to flat-screen image display devices.
In a flat-screen image display device, use is made of an electron source which is configured such that a great number of electron emitter elements are disposed on a planar substrate. Although the volume of the inside of the vacuum envelope is greatly reduced, compared to the ordinary CRT, the area of wall surfaces, from which gas is emitted, does not decrease. As a result, if the same amount of gas as in the CRT is emitted, the pressure within the vacuum envelope would greatly increase. Therefore, the role of the getter material in the flat-screen image display device is very important.
In recent years, studies have been made of forming a getter material in an image display region. Jpn. Pat. Appln. KOKAI Publication No. 9-82245, for instance, discloses a structure of a flat-screen image display device wherein a thin film of an electrically conductive getter material, such as titanium (Ti) or zirconium (Zr), is laid over a metal layer, i.e. a metal back layer, which is formed on a phosphor layer, or the metal back layer itself is formed of the electrically conductive getter material.
The objects of the metal back layer are to reflect, toward the face plate (front substrate) side, that component of light emitted from the phosphor body by electrons produced from the electron source, which travels toward the electron source side, thereby increasing luminance, to impart electrical conductivity to the phosphor layer and thus function as an anode electrode, and to prevent the phosphor layer from being damaged by ions produced by ionization of the gas remaining in the vacuum envelope.
In a conventional field emission display (FED), a very arrow gap of about 1 to several mm is provided between a face plate (front substrate) having a phosphor screen and a rear plate (back substrate) having electron emitter elements. A high voltage of about 10 kV is applied to this narrow gap, and an intense electric field is generated. Hence, there arises such a problem that discharge (vacuum arc discharge) easily occurs if an image is formed for a long time. If such abnormal discharge occurs, a discharge current of several A to several-hundred A flows instantaneously. Consequently, the electron emitter elements of the cathode section, the phosphor screen of the anode section, driving circuits, etc. may be destroyed or damaged (hereinafter referred to as “damage due to discharge”).
Recently, in order to alleviate the damage due to discharge, it has been proposed that gaps are provided in a metal back layer that is used as the anode electrode. In order to more suppress the damage due to discharge, it has been required to provide gaps in a getter film that is an electrically conductive thin film coated on the metal back layer, for example, by forming the getter film with a predetermined pattern.
As a method of forming a getter layer with a predetermined pattern, there is known a conventional method in which a mask having a proper opening pattern is placed on a metal back layer, and film formation is performed by vacuum evaporation or sputtering. In this method, however, there are limitations to the precision of patterning or to the fineness of the pattern. There is a problem that the effect of suppressing damage due to discharge is inadequate.
On the other hand, there is a method in which a dividing layer with such characteristics as to electrically divide the getter layer is disposed in advance on the phosphor screen, and the getter layer is formed and divided at the same time. The dividing layer divides the getter layer into many insular parts so that a plurality of divisional electrodes that form a metal back layer may not electrically be connected by the getter layer that is an electrically conductive film. Taking the getter layer dividing function into account, it has been thought that the dividing layer should preferably be electrically insulative.
However, as has recently become clear, when an image is to be displayed, the insulating properties of the dividing layer adversely affect withstand voltage characteristics. Electrons from the electron emitter elements are emitted toward the phosphor screen. The electrons from the electron emitter elements are made incident on the phosphor layer, and do not directly enter the dividing layer. However, dispersed electrons from the phosphor layer enter the dividing layer. If the dividing layer is electrically insulative, the dividing layer is charged with the dispersed electrons, and slight partial discharge, which leads to discharge between the substrates, may occur. It is possible that the partial discharge frequently occurs at the time of image display, and deterioration in withstand voltage characteristics may lead to degradation in image quality.